Metal powders



Patented Get. 3, 19

No Drawing. Application October 2, 1941, Serial No. 413,367

7 Claims. (Cl. 75-05) In the production of stainlesssteel, and par-'ticularly in the alloy steel casting industry, there is a considerableamount ofborings, turnings, and grinding residues availablel whi. ntainnot an inconsiderable percentage of stainless steel. These residues areproduced, for the most part, by grinding stainless steel or other highalloy castings" with an abrasive wheel. The product or grinding residuesproduced is in quite finely divided form, but due to its contaminatonwith the abrasive and bonding materialof the wheel, it has heretoforeattained nocommercial market.

In the field of powder metallurgy, stainless steel powder in variousmeshes has a considerable potential use. It has been found, as is moreparticularly explained in my copending application, Serial No. 383,875,filed March 1'7, 1941, that this powder may be compacted under suitablepressures to produce a self-sustaining compact which, with the propertechnique, may be sintered to produce an ultimate article of verysatisfactory physical characteristics.

As explained in that prior application, the stainless steel powderentering these compacts may economically be produced from stainlesssteel scrap by suitable heat treatment followed by intergranularcorrosion, ball milling or other disintegration treatment andclassification.

It has now been found that a particularly useful and economical sourcematerial for stainless steel powder or other high alloy iron powders arethe swing grinding residues produced by the final surfacing of alloycastings and similar alloy steel objects. Y

In the past this raw material, due to the relatively high percentage ofcontaminating material, has had no commercial outlet. It has been foundthat this material may be treated by simple methods to produce astainless steel or other alloy powder in desirably finely divided formand free from associated contaminants. The product thus produced isparticularly useful in certain fields, such as in the spraying of metalpowder by the Schoop gun, in which very fine material of the order of325 mesh is preferred.

The raw material which is subjected to treatment according to thepresent invention consists of grindings, borings, tumings, which containthe usual contaminants. This material contains quite a large percentageof such products as silica,

silicon carbide, organics (from the bonding agent of the grindingwheels), and the like. In most circumstances this material also containsquite a high percentage of ordinary iron, and sometimes brass.

In carrying out the invention this type of raw material is first heattreated in a special thermal range and under conditions which conduce tothe precipitation of carbides in the grain boundaries and whichcoincidentally eliminates certain of the contaminants. In this step ofthe process the raw material preferably is heat treated in a rotary kilnfurnace between 500 and 900 C. for a period varying from to 24 hours.Preferably this heating is done cyclically, that is to say, thetemperature is cyclically elevated and lowered within the carbideprecipitation range to accelerate carbide precipitation. Due to, thisheat treatment carbides are precipitated in the grain boundaries andslip planes; coincidentally, a I considerable amount of contaminantssuch as sulfur and the like are burnt off.

The product produced by the heat treatment is then quenched in asuitable contact corroding medium such as in a spent pickling liquor,from steel pickling, to which liquor has been added of the order of 5%of sodium fluoride and sometimes up to 5% sulphuric acid. In thusquenching the hot material a considerable amount of the adherentabrasive is detached from the metal particles by exfoliation. Due to thecontact with the pickling liquor additional amounts of the abras ves aredetached by reason of the under-cutting effect of the pickle. Since theheat treated material is quenched in the pickle liquor the temperatureof the latter is quickly elevated and the pickle is extremely active.

After the quenched product has been retained in the pickle liquor for apredetermined period of time, usually from about 30 minutes or more, theliquor is separated from the solid residue. This may be done in anysuitable continuous filtering apparatus or by decanting off thesupernatant pickle liquor.

The residue from the pickling stage may be washed to remove adherentpickle'and is then screened. It has been found that by a simplescreening method a very large proportion of the oxides and abrasivespresent in the original residue can be removed. Specifically, it hasbeen found that most of these oxides and abrasives are separated fromthe metallic particles by screening through approximately a or 200 meshscreen and discarding the 100 or 200 mesh material. It has been found,similarly, that much of the low iron alloy content in the originalresidue will be removed in this screening operation, due to the factthat the low alloy iron is corroded to fines during the pickling,whereas the substantially corrosive-resistant stainless steel particlesare unattacked. I

The coarse material produced from the above screening operationcomprises essentially stainless steel or other high alloy iron powderand is then disintegrated. This may effectively be done by ,crushingthe'material in a suitable type of hammer mill and preferably in amicro-pulverizer. This type of machine, as is known, can be set for amore or less definite degree of disintegration, i. e., for -100, -200,and 300 mesh material. The micro-pulverizer is peculiarly effective forthis operation, for the material can be disintegrated at the rate ofabout 25 to 400 pounds per hour. In this type of apparatus, as thoseskilled in the art recognize, a series of stellite hammers. move througha small distance, of the order of a! of an inch, at extremely highspeed. It will be understood that in this disintegrating operation thematerial to be treated may be fed to the pulverizer either wet or dry orin admixture with any suitable lubricant. During this disintegratingoperation, due to the cold work applied to the metal, the alloys treated(18-8; 18-8-Mo; 18-8-Ti; etc.) are rendered ma netic.

The crushed material produced in the pulverizer step may then be treatedin a suitable corrodent, so as to selectively attack the grainboundaries (high in carbides) and for removing imbedded abrasive scale,low alloy iron and the like. The corrodent for this step of the processmay comprise a suitable aqueous solution of acid ferric sulphate andsodium fluoride. The period of retention in the corroding solution willvary depending upon the particular product treated, the temperature ofthe corroding solution, and the like.

The crushed and corroded material is now re- 'moved from the corrosionvat and is washed to free it of adherent corrodent. The product is thenconcentrated, preferably on a Welfiey table, to separate the metalparticles from the nonmetallic particles, i. e., abrasive material,oxides, carbides and the very fine unalloyed iron. These latter productsare discarded in the tailings and the concentrate is recovered, whichcontains of the order of 90 to 98% of stainless steel powder.

If desired, the tailings, middlings and concentrate produced in theabove concentration step may be further concentrated either byflotation, electrostatic or magnetic separation.

The final concentrate, which comprises essentially an 18-8 steel powder,is then contacted or washed with a hot nitric acid solution betweenabout and 20% concentration. This treatment serves a number of usefulfunctions, namely, to brighten or shine such powder. The material thusproduced, as noted previously, is particularly useful for spraying inmetal guns of the type mentioned because of its desirably fine mesh, andfor such purpose the product can be used directly.

The product. may also be used for powder metallurgy purposes. However,for this use it is necessary to subject the material to an additionaltreatment, preferably to a dry hydrogen anneal in a temperature rangebetween about 900 C. and 1200 0.

As will be appreciated by those skilled in the art, the heretofore wastematerial, namely, the,"

grinding residues, may be processed to produce valuable alloy steelpowder by modifications of the above method. Useful discrete steelpowder may be produced from the described source material by firstcrushing the raw material in a micropulverizer, or similar machine, tosuch a degree that it passes through a 200 mesh screen. Since thesemetal particles are already quite severely work hardened (due to thegrinding and boring operations which produce them) they are quitebrittle and are readily disintegrated. Durina this grinding operationthe contaminants in the raw material, 1. e., grinding oil, sulphur,rubber, Bakelite and the like, serve beneficially in the role oflubricants. During this grinding operation the attached or adheredabrasive particles are largely removed from the mesh by impact.

The ground product described above is then fed to a screen whichclassifies the material into two fractions, one of a screen analysisbetween 200 and -325 mesh and the other a -325 mesh. These two fractionsare then each passed through a magnetic separator, or if desired eachfraction may be treated by gravity separation methods such as flotation,Welfiey table separation, and the like.

The magnetic concentrate, or the concentrate produced by table orflotation, is treated .with a nitric acid solution of suitableconcentration. This treatment serves to degrease the product byundercutting and to passivate the stainless steel. If the product is tobe used for a remelting addition agent, in lieu of passivation,'it maybe treated with a typical pickling solution which serves to dissolve thesulphur and levigate the extreme fines of abrasives, oxides, carbides,Bakelite, and the like. This scum may then be removed from the top ofthe pickle bath.

The material thus produced is washed and dried and may be briquetted inany suitable manner, so as to be employed in a high alloy steel melt.

If the product is to be used as a pigment, for example, in paints, it ispreferably ball milled either in a wet ball milling operation in contactwith a liquid medium such as Varsol or it may 'be dry milled andpolished with stearic acid.

It is to be observed that if the product from the magnetic separator orother concentrator is too high in iron the nitric acid treatment or thepickling is continued so as to dissolve ordinary iron and steel.

If the material produced is to be used for Y spraying, it may be mixedwith a flux or with a of homogeneously incorporating the flux or the lowmelting metal powder in the stainless steel is to run the mixturethrough the micro-pulverizer. Another method is to separately grind thefiux or low melting point metal powder so that its particle size will beequal to and preferably less than that of the stainless steel powder,and then mixing the two. For 18-8 spraying powders, in

addition to copper and the iron eutectic powder.

mentioned the low melting calcium-manganesesilicon alloys,silver-copper-phosphorus eutectic alloy, term-manganese and the like,may be utilized. Any metallic product compatible with stainless steelpowder and which serves to accelerate or facilitate the bonding ofthesprayed powder to the base material may be used.

After spraying the improved powder on the base material, for example ona low carbon steel base, the sprayed coating may be wire brushed tosecure a desired matte finish, or the material can be abraded fiat andthen brushed, and if desired finished bypolishing. Similarly, thesprayed material may be electrolytically polished.

In some circumstances, where an extremely tenacious bond between thesprayed surface and the base plate is desired, it is advantageous toanneal the article under bright annealing conditions.

Spraying powder of the type above described is useful in a wide varietyof fields, as, for example, in the application of corrosion-resistant,relatively fine-sectioned surfaces to more corrodible base metal, and tothe building up of steel surfaces, by the sprayin technique, to adesired specification. In the event that highly wearresistant coatingsare desired, the applied sprayed coating may be suitably treated, as byflame hardening, or high frequency hardening, to improve the wearingqualities of the applied coat.

I claim:

1. As a new article of manufacture an alloy steel powder of the 18-8type particularly suitable for metal spraying which consists essentiallyof a homogeneous mixtureof preponderant percentage of] 18-8 powder and aminor percentage of a lower melting point metal such lower melting pointmetal being of a kind which improves the adherence of the sprayed 18-8powder to the base material on which it is sprayed.

- the lower melting point'metai powder being of the type which improvesthe adherence of the sprayed 18-8 powder to the base material on whichit is sprayed. g

5. As an article of manufacture a metal spraying powder consistingessentially of 18-8 powder in homogeneous admixture with a low meltingalloy powder containing calcium, manganese and silicon.

6. As a spraying powder, 18-8 powder in homogeneous admixture with aminor percentage of a low melting alloy of silver, copper andphosphorus.

7. As a spraying powder, 18-8 powder in homogeneous admixture with aminor percentage of a low melting alloy of silver, copper andphosphorus, and having a screen analysis of --325- mesh.

JOHN WUII'F.

